Modern, high density farming frequently employs the use of a plastic film mulch, such as polyethylene, as a covering or partial covering for fumigated beds. The mulch retains moisture and controls weed growth. Much of this farming is done in warm climates. In many of these climates, multiple plantings are feasible except for a few rare days of frost or freezing temperatures. However, when frost does occur it is devastating. Entire crops can be lost. To the farmer this can result in losses in excess of thousands of dollars per acre. There is a tremendous need for a cost effective method of protecting winter crops during these infrequent cold spells.
There are several prior art techniques used to minimize damage. One of the more popular methods is to water the crop continuously with overhead sprinklers. This method works at freezing temperatures except when the cold is accompanied by wind. The wind redirects the distribution of water allowing sections of the crop to go unprotected. Increasingly, farmers are using trickle irrigation to conserve water and in that case no sprinkler system is available. Smudge pots are another protective measure. They produce a heavy smog which can keep the area warm in the absence of wind.
A still newer method is the use of water containing fertilizer salts which, when absorbed by the plants, act as a freezing point depressant. This method is expensive and controversial. Even more controversial is the use of man-made bacteria which contribute to frost protection.
One of the most effective methods of protecting the plants is to cover them with a film such as clear polyethylene, However, daylight temperatures under such film can reach temperatures that cause the plant to die, or in some respects worse, live but produce low crop yields.
In order for any protective method to be commercially effective it must be adaptable to automation. Typically, frost warnings are forecast a maximum of 48 hours in advance. Unless the protective method is automated, there is insufficient time to protect the fields. In many instances, strong winds accompany severe weather changes. The wind can be nearly as injurious as the frost. The protective method must therefore also be immune to those winds.
If a protective covering is used, it must have the capability to remain on the bed 5-10 days. The covering must be able to withstand weather for that period, and the environment under the covering must be conducive to maintaining the plant's subsistence. It is unlikely that a single type of covering will be applicable to all plants. The criteria varies even within the same species. For instance, spun bonded nonwovens have been tested as a covering for strawberries. It was found that strawberries which have most of their blooms on top of the plant yielded deformed berries. This is believed to have been due to the abrasive action of the covering rubbing against the buds. Another variety of strawberries having most of its blooms on the bottom of the plant had a lower percentage of deformed berries. Also, the covering must be economical enough that the incremental increase in cost is justified.